UITextField限制字数的方法

在输入东西的时候，如果想限制最大字数，可以用下面方法

  - (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string; 
    { 
        if ([string isEqualToString:@"\n"])  
        { 
            return YES; 
        } 
        NSString * toBeString = [textField.text stringByReplacingCharactersInRange:range withString:string]; 
         
        if (self.myTextField == textField)  
        { 
            if ([toBeString length] > 20) { 
                textField.text = [toBeString substringToIndex:20]; 
                UIAlertView *alert = [[[UIAlertView alloc] initWithTitle:nil message:@"超过最大字数不能输入了" delegate:nil cancelButtonTitle:@"Ok" otherButtonTitles:nil, nil] autorelease]; 
                [alert show]; 
                return NO; 
            } 
        } 
        return YES; 
    }